Conventional electrochemical cells, such as alkaline cells, are formed of a cylindrical housing having an open end. After the cell contents are supplied, the cell is closed by crimping the housing edge over the end cap assembly to provide a seal for the cell. The end cap assembly comprises an exposed end cap plate which functions as a cell terminal and typically a plastic insulating member which insulates the end cap plate from the cell housing. A problem associated with design of various electrochemical cells, particularly alkaline cells, is the tendency of the cell to produce gases as it continues to discharge beyond a certain point, normally around the point of complete exhaustion of the cell's useful capacity. Electrochemical cells, particularly alkaline cells, are conventionally provided with rupturable diaphragms or membranes within an end cap assembly for the cell's casing. The rupturable diaphragm or membrane may be formed within a plastic insulating member as described, for example, in U.S. Pat. No. 3,617,386. Such diaphragms are designed to rupture when gas pressure within the cell exceeds a predetermined level. The end cap assembly may be provided with vent holes for the gas to escape when the diaphragm or membrane is ruptured. The end cap assembly disclosed in this reference uses considerable space above the rupturable diaphragm which reduces the amount of available space within the cell for active material. Also, the end cap assembly disclosed in the reference is not designed to withstand radial compressive forces and will tend to leak when the cell is subjected to extremes in hot and cold climate.
In order to provide a tight seal the prior art discloses end cap assemblies which include a metal support disk inserted between the end cap plate and an insulating member such as a plastic grommet which electrically insulates the metal support disk from the cell casing. The metal support disk may have a convoluted surface as shown in U.S. Pat. Nos. 5,532,081 or 5,080,985 which assures that end cap assembly can withstand high radial compressive forces during crimping of the cell's housing edge around the end cap assembly. Such support disk allows the radial forces to be maintained. This results in a tight mechanical seal around the end cap assembly at all times.
Also, the prior art discloses rupturable vent membranes which are integrally formed as part of an insulating member included within the end cap assembly. Such vent membranes are typically in the form of a rupturable disk which lies in a plane perpendicular to the cell's longitudinal axis, for example, as shown in U.S. Pat. No. 4,537,841. As shown in this reference the rupturable disk shaped membrane is integrally formed as a thin portion forming a part of an insulating grommet. Also, as shown in this reference, there is sufficient free space above the rupturable disk to allow the disk to rupture cleanly and allow gas to escape there through. The rupturable thin portion within the insulating member may also take the form of a circumferential vent membrane integrally formed within the insulating grommet as disclosed in U.S. Pat. No. 5,080,985. The circumferential membrane forms the thin portion of the insulating grommet. Such membrane may have a grooved edge to facilitate rupture of the membrane when gas pressure within the cell exceeds a predetermined value. Such rupturable vent membranes whether disk or circumferential in shape or in the form of grooved thin portions are characterized by being supported by heavier, thicker structure of the insulating grommet which immediately surrounds the rupturable membrane portions.
Recently condition testers for electrochemical cells, for example alkaline cells, have been integrated into the label for the cell to form a label/tester composite which is attached to the cell housing. The condition tester may typically be a thermochromic tester, but alternatively it may be an electrochromic tester, electrochemical tester, coulometric tester or equivalent which is attached to the inside surface of the label. The condition tester may have an electrically conductive layer therein. When the ends of the conductive layer are pressed into contact with the cell terminals the conductive layer reaches an equilibrium temperature which is a function of the cell voltage. If the conductive layer becomes hot enough it causes a thermochromic layer of the tester to change appearance thereby giving the viewer a visual indication of whether the cell is strong or weak. A label/tester composite employing a thermochromic type tester for attachment to the cell housing is described in U.S. Pat. Nos. 5,612,151 and 5,614,333.
When the label/tester composite is to be applied to conventional alkaline cells, one end of the conductive layer must be either permanently electrically connected to the terminal end cap or else allowed to be manually pressed into electrical contact with the end cap. In U.S. Pat. No. 5,614,333 an embodiment of the label/tester is shown wherein an end of the conductive layer is intended to be manually pressed into contact with the terminal end cap. That conductive end is separated from the terminal end cap by an electrically insulating layer having apertures therethrough. To activate the tester the conductive end is manually pressed through these apertures to contact the terminal end cap by applying finger pressure to the label portion thereover. A ring may be inserted as a separate piece between the peripheral edge of the terminal end cap and the cell's housing to provide a contact platform for the conductive layer or leads emanating therefrom as described in U.S. Pat. No. 5,491,038. Alternatively, an end of the tester's conductive layer may be permanently secured to the cell's terminal end cap using a conductive adhesive as described in U.S. Pat. No. 5,543,246. Commonly assigned U.S. patent application Ser. No. 08/897,918 filed Jul. 21, 1997 discloses a terminal end plate with a nonuniform surface and having a flat portion to which may be connected the conductive layer of a condition tester integrated into the label for the cell.
Accordingly, it is desirable to have an end cap assembly which provides a tight seal for the cell even though the cell may be exposed to extremes in both hot and cold climate and wherein the end cap assembly occupies a minimal amount of space within he cell so that additional active material may be added to the cell to increase capacity.
It is also desirable to have a rupturable venting mechanism which occupies minimal amount of space within the cell and which can be readily manufactured so that venting occurs at a desired predetermined pressure.
It is desirable to have a terminal end cap with a portion of its surface of structure providing good electrical contact with the conductive portion of a label/tester composite regardless of whether permanent or manual contact is desired.